Painting with predefined materials in 2.79?
Here I am using a black image and painting white into it using a brush mask set to rake and random, and that is plugged into the mix factor of the mix shader between a Principled shader and a diffuse shader.

The diffuse is a brown to represent the rust and the principled is only slightly metallic for the outside paint/metal layer. The bump node helps sell the difference between the two shaders, and if you go about painting a color image for the rust and a color for the paint, you can get some really nice effects.

I made some brushes that are on Blendswap for this actual process. To simplify it, you can use the same process but run the texture into a color mix node, using the mix factor to determine the difference between the two color inputs (images) and plug that into a single shader, still using a bump node into the vector of the shader.